I will be designing a book review website for a client where he will be writing book reviews (both favourable and unfavourable) of novels in his genre. Can we legally use an image of the book cover (downloaded from somewhere like Amazon) to make our site seem more attractive or do the publishers retain copyright of this image? I have seen other review sites that use book cover images but am not sure of the legalities.
Using the book cover as part of a review would constitute fair use. You're allowd to replicate a small percentage of the text too - for example, to quote a small snippet from a chapter if it's especially interesting. What you're not allowed to do is copy huge chunks of the book. You can see some examples of what is, and is not, fair use, here: http://fairuse.stanford.edu/Copyright_and_Fair_Use_Overview/chapter9/9-c.html
it's ok to use the cover image of the book because you are just showing it to the readers what exactly that book looks like. You can also add a short caption under the book cover image the source/website where you copied it. you will only get in trouble if you are owning others work.
You're fine as you need to show what it is you are reviewing. Same would apply to any product being reviewed, such as cars, computers, etc.